The present invention relates generally to gas turbine engines, and, more specifically, to turbine shrouds.
A typical turbofan aircraft gas turbine engine includes a fan powered by a low pressure turbine (LPT) and a multistage compressor powered by a high pressure turbine (HPT). Ambient air flows in turn through the fan and compressor which provides compressed air to a combustor wherein it is mixed with fuel and ignited for generating hot combustion gases which flow in turn through the HPT and LPT which extract energy therefrom.
The turbines are defined by rotor blades extending radially outwardly from corresponding disks cooperating with stationary turbine nozzles disposed upstream therefrom. The nozzles direct the combustion gases between the turbine blades which extract energy therefrom for rotating the disk. Circumferentially surrounding the radially outer tips of the turbine blades is a stationary turbine shroud which defines a relatively small radial gap therebetween for reducing the amount of combustion gas leaking therethrough. The blade tip clearance should be as small as possible for maximizing efficiency of the engine.
The turbine shrouds are typically air cooled using a portion of compressor discharge air from the compressor outlet which is channeled to the radially outer or backside of the turbine shrouds. The turbine shrouds therefore experience a differential temperature between the hot combustion gases which flow along the radially inner surface thereof and the cooling air provided on the backside thereof. This differential temperature affects the blade tip clearance over the various power settings of the engine, and therefore the turbine shrouds must be suitably supported for minimizing blade tip clearance variation.
In practice, the turbine shroud is an assembly or system of discrete turbine shroud segments circumferentially adjoining each other. Each segment includes forward and aft hooks which engage corresponding forward and aft hooks of an annular shroud hanger mounted in turn to a surrounding annular shroud support. The main purpose of the shroud hanger is to support the shroud segments and isolate the shroud support from the hot shroud itself. The shroud support generally controls the radial growth of the shroud segments and, therefore, the blade tip clearance.
The shroud is typically cooled by the high-pressure compressor discharge air channeled thereto through apertures in either the hanger or the shroud support. An impingement baffle is typically provided on the backside of the shroud through which the cooling air is channeled for impingement cooling the shroud segments. The impingement air may then be channeled through the individual shroud segments or may leak around the edges of the shroud segments back to the main flowpath through which the combustion gases flow. For example, the individual shrouds may include inclined film cooling holes extending therethrough to provide a layer of film cooling air along the inner surface of the shroud for protection against the hot combustion gases flowing thereover during operation.
The purpose of the turbine rotor blades is to extract energy from the combustion gases which in turn creates a pressure drop over the blades. Accordingly, the gas pressure is highest at the leading edge of the blades near the forward end of the turbine shroud, and lowest near the trailing edge of the blades adjacent the aft end of the turbine shrouds. The gas pressure therefore varies axially between the forward and aft ends of the shroud.
This in turn increases the difficulty of effectively cooling the turbine shroud since its cooling air is provided at a single pressure. The differential pressure radially across the turbine shroud therefore increases from its forward end to its aft end which undesirably increases cooling air leakage and film cooling hole blowing ratio. Engine efficiency therefore suffers since any excess cooling air diverted to the turbine shroud does not undergo combustion for producing power during operation.
Accordingly, it is desired to provide a turbine shroud having improved cooling resulting in increased engine efficiency.